The objective of the study was to examine the effect of the homeopathic remedy Betula 30c vs. placebo for patients with birch pollen allergy. A double-blind, randomized, placebo-controlled trial was carried out. Tablets were given for 4 weeks during the birch pollen season. The setting was Oslo, Norway, May 1995. Patients were aged between 18 and 50 y; 32 patients received Betula 30c tablets and 34 patients received placebo tablets. The main outcome measure was the total score of 17 different allergy symptoms. Daily total scores were calculated, as well as differences and ratios between the run-in and the following time periods. Point estimates of the median difference between the experimental and placebo groups, with their 95% confidence intervals, were the main measure of effect. No statistically significant difference between the groups was found during the first and last period of May. However, from 8 to 18 May, a clinically interesting difference was revealed between the groups, those receiving Betula 30c having fewer and less serious symptoms. For some days these differences were statistically significant. Surprisingly, this group reported more aggravation from the tablets than did the placebo group. With a statistical power of 70% for a defined clinically interesting difference (25%), the present results indicate that treatment with Betula 30c during the pollen season deserves further attention.